The dramatic progress of computer technology over the past four decades has been fueled by an unparalleled development of the three basic hardware elements: storage devices, processors and displays. In all three areas, the art and science of surface structuring is of essential importance. The trends towards miniaturization and integration as well as towards improved performance. reliability, and productivity require increasingly better control of surfaces and interfaces down to the molecular or atomic level.
Today. design rules for integrated circuits and storage devices have become so small that some of the most important conventional structuring techniques got close to their principal limits. Standard optical lithography, for instance, is bound to dimensions larger than about one half wavelength which amounts to about 140 nm for uv radiation. The bitsize of CD-ROMs is limited to about 0.5 micron because present-day light-emitting diodes radiate red light with wavelengths of 800 to 1000 nm. Obviously, new techniques are needed in order to further reduce the design rules.
With regard to display technology, the trend towards flat panel designs generates an increasing need for large, matrix addressable arrays of pixel elements. Here, the technical problem is not miniaturization but energy efficiency. yield and cost. At present, liquid crystal displays (LCD) play the major role in flat-panel technology. The LCD cell changes its optical transmissivity by appropriate application of voltage. Excellent contrast is achieved if arrays of thin film transistors (TFT) turn on and off the individual LCD cells. This scheme however. is costly and TFT-LCDs are correspondingly expensive. Moreover, LCDs quite generally suffer from poor energy efficiency since they have to be illuminated by an external light source, the different colors resulting from filtering of white light. Because of these short-comings, the display industry is greatly interested in potential alternatives to the LCD technology.
Light-emitting diodes (LED) are much more energy efficient than LCDs but the presently available LEDs are too expensive for large-scale display panels. Present R&D activities in this field therefore concentrate in particular on organic LEDs (OLED) which hold promise for low-cost mass production. An overview about the design and properties of OLEDs can be found in the article by J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking "Organic Electroluminescent Devices" which appeared in Science Vol. 273, p. 884, 1996. In principle, OLEDs consist of layers of polymers sandwiched between a metallic and a transparent semiconducting electrode. The main short-coming of OLEDs is presently a short lifetime because of chemical interactions with the environment and degradation due to electron injection at too high energies, necessitated by a poor match of the electronic properties of the polymers with respect to the electrode materials.
It is possible, however, to influence the electron injection capability by modification of the molecules next to the electrode/polymer interface. A. Haran, D. H. Waldeck, R. Naaman, E. Moons and D. Cahen describe in "The Dependence of Electron Transfer Efficiency on the Conformational Order in Organic Monolayers", Science Vol. 263, pp. 948-950, 1994, for instance, the influence of a monolayer of octadecyltrichlorosilane (OTS) molecules on the electron transfer (ET) process from a silicon electrode into an aqueous electrolyte solution.
The OTS layer can be switched by heating from one conformation into another. The different conformations correspond to different geometries of a given molecule arising from bending or flexing of certain molecular bonds.
The authors found that current-voltage characteristics depend not only on the degree of coverage but also on the conformation of the OTS molecules. In one example, the reverse current at a voltage of 0.2 V changed from 4.5 to 0.2 microamperes when the layer was transformed from the one into another conformation.
Research towards the controlled fabrication of small structures has been greatly inspired by the success of scanning probe microscopes (SPM) in recent years. Numerous examples confirm that the pointed probe tip of the SPM not only is able to monitor variations in the sample surface structure with atomic or near-atomic resolution but that it also can be used to modify the surface on a similar scale. It was demonstrated by T. A. Jung, R. R. Schlittler, J. K. Gimzewski, H. Tang, and C. Joachim in Science, Vol. 271, p. 181, 1996, for instance, that individual molecules can be moved into prescribed fixed new positions and/or be modified without change of position under the influence of the SPM tip. In the pursuit of such investigations it was found that molecular flexibility plays a crucial role for such manipulations.
Fixed ordered molecular layers on a substrate are generated, e. g. by molecules forming Langmuir-Blodgett (LB) or self-assembled monolayer (SAMI) films or undergoing cooperative self assembly or being deposited by sublimation or by conformational epitaxy.
The molecules can exist in different conformations which are characterized by the (meta-) stable orientations and/or positions of the different entities of which they are made up. The different entities of these molecules consist of individual atoms or molecule-like sub-entities of atoms which are more strongly bound among each other than to the atoms of the other entities. Frequently, the connections between individual entities arc single molecular bonds which can act as axis for a relative rotational motion of the entities. Switching between different conformations usually comprises such rotational realignment of entities. Molecules structured into such entities are standard in organic chemistry.